1. Field of the Invention
This invention is related to positioning apparatus and, more particularly, to positioning apparatus for positioning an object with respect to two lateral positions.
2. Description of the Prior Art
In certain applications, it is often desirous to position an object with respect to two spaced positions. These applications include, for example, article handling devices such as inspection or other type probe devices, drills, workpiece carriers, etc., and the like.
One particular application, for example, known in the prior art is an automatic sample testing machine used for testing radioactive samples, which are contained in test tubes, within the well of a well-type scintillation counter. This prior art machine employs conveyor means which positions a test tube gripper assembly or mechanism from a test tube gripper station to the scintillation counter well station and back. The conveyor means includes a twin pair of endless sprocket chains substantially aligned in parallel vertical planes. Each chain is entrained about a set of five sprocket wheels such that each chain is provided with a pentagonal-like configuration with elongated reached or sides between the sprocket. Corresponding sprockets of the two sets are aligned with each other so that the two chains have identical aligned configurations.
The positioning apparatus of this prior art machine has a tranversely extended shaft rotatably carried by two brackets, each of which is mounted to one of the two chains. Mounted on the extended shaft is the test tube gripper assembly so that the latter is supported thereby and between the two chains in a straddled manner. The twin chains' composite pentagonal configuration is such that it has two opposing vertical sides, a shorter one of which is associated with the test tube gripper station and the longer one of which is associated with the well station, a top horizontal side connecting the upper ends of the two vertical sides, an opposing somewhat shorter bottom horizontal side which is connected at one end to the lower end of the short vertical side, and an inclined side which connects the other end of the shorter horizontal side to the lower end of the long vertical side.
In operation of the prior art machine, the test tubes are carried on an indexable annular horizontal turntable which indexes the turntable to the gripper station. The superimposed gripper mechanism, when actuated at the gripper station, grips a test tube so that the conveyor means carries it and the tube gripped thereby first vertically upwards along the short vertical side withdrawing the tube from the table carrier, then along the upper horizontal side, and then vertically downward along the long vertical side where the tube is inserted into the scintillation counter well, which in the particular prior art machine, is located below the center opening of the annular turntable. To withdraw the tube from the wall after the test has been completed and return the tube to the carrier turntable, the direction of the twin chain is reversed.
The positioning apparatus of the prior art are complex, and generally unreliable and/or unstable, particularly for applications where positioning of the object is critical. For example, in the prior art machine previously described, the elongated reaches of the chains are subject to sagging which is not fully controllable by tensioning sprockets which are provided for this purpose. As a result, should the test tubes be intentionally or unintentionally loaded to capacity with the specimen, spillage is likely to occur due to swaying of the freely suspended gripper mechanism and/or of the chains caused by the instability of the sagging chains. For similar reasons, the sag in the chains makes accurate positioning of the object at each of the positions unreliable so that the prior art machine is not suitable for applications where accurate positioning is critical. Moreover, in the particular prior art machine aforedescribed, these disadvantages are further compounded by the requirement therein of using two chains, since each chain is susceptible to sagging by different amounts.